The invention relates to an acrylate hot melt-based self-adhesive composition having a K value of at least 60.
Acrylate hot melt-based self-adhesive compositions are known, but these are unsuitable or insufficiently suitable for many applications due to their low K value. There are therefore commercially available compositions of this type in the form of low-viscosity systems, for example as HRJ 4326 (Schenectady Chemicals) having a K value of 31 (Beiersdorf test method) and a viscosity of 3.8 Paxc2x7s at 177xc2x0 C. (350xc2x0 F.), and ACRONAL DS 3429 X (BASF) having a K value of 40 and a viscosity of 13 Paxc2x7s at 120xc2x0 C. However, these and similar compositions are unsuitable for coating woven and nonwoven materials, for example as supports for self-adhesive tapes or plasters, since they pass through such supports. In addition, their adhesive strength is too low, in particular, for technical applications.
The restriction of known compositions of this type to low-viscosity systems is attributable, in particular, to the fact that the requisite concentration step places high demands on the flow behaviour and the resistance to heat and shear. The adhesive compositions, which are prepared in solution, are therefore usually concentrated directly in the reactor. This operation requires:
A low viscosity of the system, even in the highly concentrated state, in order to allow effective degassing and mixing.
A low viscosity of the compositions to allow economical removal thereof from the reactor.
Using the same technology, compositions of higher viscosity would have the following disadvantages, inter alia:
Higher viscosity systems would result in a significantly increased demand for thermal energy here, which would result in increased heating of the compositions (side reactions/decomposition).
The use of lower temperatures would result in a correspondingly high shear stress of the composition systems.
For the above reasons, it has hitherto not been possible to prepare acrylate hot-melt adhesives having higher K values, but compositions of low viscosity have adhesion-related weaknesses as far as shear strength is concerned.
The object of the invention was to provide a remedy here, in particular to provide acrylate hot melt-based self-adhesive compositions which have a K value of at least 60, in particular 65-80.
Such compositions are not commercially available, in particular because there is no industrially practicable and appropriate process for their preparation, despite the fact that there is considerable interest in such compositions and extensive development work being carried out to increase the K values of the compositions available hitherto.
This object is achieved according to the invention by the compositions and their preparation as characterized in greater detail in the claims.
The use of a devolatilizing (vented) extruder for the purposes of the invention was not obvious to the person skilled in the art for a number of reasons; it would have been strongly expected that high K values of the acrylate hot melt, such as those of at least 60, would result in degradation of the macromolecules due to shear forces and heat treatment competing with the devolatilization, gelling owing to reactive co-components, and fundamental reservations against an excessive solvent content, as is now apparent in the form of a prejudice against the use of a devolatilizing extruder for high-molecular-weight adhesive products of this type. Although it has already been disclosed, in European Patent 411 510, to remove volatile constituents from a polymer solution in the extruder, as occurs, in particular, for the removal of residual monomers. This European Patent nevertheless, as conventional in the prior art, uses various thermoplastics, such as polycarbonate, polyphenylene ether or polystyrene, which are not comparable with the acrylate hot melt having a high K value which is employed according to the invention. The reason for this is that the person skilled in the art has a prejudice against conveying adhesive compositions in a devolatilizing extruder.
Preference is given according to the invention to the measures mentioned in the sub-claims. The solvents used are, for example: gasoline, acetone, ethyl acetate, toluene, C1-C5-alcohols, C5-C9-alkanes and mixtures thereof. The concentration can be carried out by additionally using suitable measures, such as the use of entrainers, for example water, to assist with the devolatilization process. In addition, the vacuum can preferably be broken using a protective gas, such as nitrogen or argon. It is particularly favourable if the extruder used has one or preferably more independently heatable zones. The conveying capacity is preferably matched to the acrylate hot melt to be processed in each case by adjusting the rotational speed and the screw configuration.
Advantages are thus achieved according to the invention which were unforeseeable by a person skilled in the art.
Even compositions having high K values can be devolatilized while protecting the material, since the temperature and shear rate can be matched to requirements. Restrictions with respect to formulation possibilities have been overcome compared with low-viscosity systems.
Flowing of the compositions at the 100% stage is not absolutely necessary due to the mechanical conveying.
In contrast to the batch procedure, continuous production is possible.
The compositions having high K values also satisfy the requirements of an industrial adhesive tape.
The shear strength of these systems are higher, and the requisite processing effort is lower.
The invention is described below with reference to illustrative embodiments, without wishing unnecessarily thereby to restrict the invention.